Electric vehicles and internal combustion engine powered vehicles may be powered by a number of different fuels. Internal combustion engine powered vehicles may be powered by various fuels including gasoline, diesel, ethanol, methane, or hydrogen, for example. Fuel cells have been proposed as a power source for electric vehicles, and other applications. Such a fuel cell system is disclosed in commonly owned U.S. patent application Ser. No. 10/418,536, hereby incorporated herein by reference in its entirety. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied as a fuel to an anode of the fuel cell and oxygen is supplied as an oxidant to a cathode of the fuel cell. A common technique for storing large quantities of hydrogen is to cool and compress hydrogen via liquefaction techniques, and to store the liquid phase hydrogen in a cryogenic storage tank. Hydrogen gas liquefies at −253° C. and can be stored at about 70 g/L in the liquid phase. The amount of energy required to cool down hydrogen gas into a liquid is very high, and currently may use as much as 40% of the energy obtained from the hydrogen fuel. Thus, it is advantageous to keep the liquid phase hydrogen insulated to militate against liquid evaporation.
Any transfer of heat to the innermost portion of the cryogenic storage tank affects the natural evaporation rate of the cryogenic vessel. The more heat that is transferred, the faster the rate of boil-off of the liquid hydrogen, or the higher the natural evaporation rate. In order to maintain the hydrogen in a liquid state, heat transfer from the ambient environment to the cryogenic liquid must be kept to a minimum. Cryogenic storage tanks generally consist of an inner storage vessel encapsulated with an outer vessel or shell. The space between the inner vessel and the outer vessel is commonly well insulated and maintained under a vacuum. An interior of the inner vessel, however, must include fluid communication means, typically in the form of inlet and outlet conduits, for the filling and extraction of liquid and gaseous hydrogen.
A typical storage tank includes a liquid inlet conduit, a liquid outlet conduit, and an inlet and outlet gas conduit. The liquid inlet conduit and the liquid outlet are sometimes combined into a single conduit. Further, additional conduits are sometimes included to provide a path for cables to sensors or heaters that may be included in the inner vessel. The three conduits typically penetrate a sidewall of the storage tank through three separate apertures, or together in a common vacuum tube penetrating the sidewall of the inner vessel. At least a portion of each conduit is exposed to the ambient environment. The conduits bridge an insulation that is present between the inner vessel and the outer vessel, and allow parasitic heat from the ambient environment to transfer into the inner vessel.
The use of a vacuum tube is a typical method employed to mitigate the heat transfer from the ambient environment to the inner vessel. A vacuum tube is provided that extends into the inner vessel creating a tubular cavity. The inlet and outlet conduits pass through the vacuum tube before penetrating the inner vessel. The cavity in the vacuum tube is maintained colder than the inlet and outlet conduits contained therein. The colder temperature in the cavity cools the inlet and outlet conduits, and reduces the heat transfer by the inlet and outlet conduits from the ambient environment into the inner vessel.
The use of the vacuum tube has some shortcomings. The vacuum tube reduces a storage volume of the inner vessel. Further, testing the inner vessel for vacuum tightness once welded closed is difficult and any repairs to welds or conduits at a far end of the vacuum tube are not possible. Accordingly, there is a need for an improved cryogenic liquid storage tank and particularly, one that minimizes heat transfer originating from the inlet and outlet conduits and maximizes the storage volume and serviceability of the inner vessel.
It would be desirable to develop a cryogenic storage tank with a minimized heat transfer originating from the inlet and outlet conduits and maximized storage volume and serviceability of the inner vessel.